Erosion-resistant turbine blade



April 17, 1962 G. w. SCHEPER, JR

EROSION-RESISTANT TURBINEBLADE Filed Sept. 22. 1959 fm/enior Gear MK Sche oe/"J/r by 4 I W H/s Affo/"ney particles entrained in the steam flow.

snasnrt Patented Apr. 17, 1962 free 3,039,071 EROSION-RESISTANT TURBINE BLADE George W. Scheper, in, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Sept. 22, 1959, Ser. No. 341,485

5 Claims. {CL 253-39) This invention relates to improved turbine blading for utilizing a motive fluid which contains entrained solid or liquid particles conducive to blade erosion, and more particularly it relates to a rotor blade structure for use in gas turbines burning fuels containing substantial ash in the otherwise gaseous products of combustion.

Of the three primary fuels generally used in powerplants, natural gas, fuel oil, and coal, the latter is the least expensive, and yet has been the last to be adapted for use in gas turbines. Coal presents many problems, not the least of which is the erosion caused to the gas turbine parts due to particles of ash contained in the products of combustion. Ash is the non-combustible residue remaining after complete combustion of the coal, and at the flow velocities used in gas turbines, it can cause severe erosion when it impinges upon a surface disposed generally normal to the gas flow. Experimental tests have indicated that one of the most persistent and at the same time one of the most serious places that this can occur is at the root of the vane portion of the rotor blade, since the erosion of the blade cross-section at this point causes stress concentrations. With a weakened vane root, centrifugal force can cause the blade to break with severe damage to the remaining turbine blading.

A similar situation exists at the root of the stator vane where it joins the casing, although the situation is not quite as serious here since the stator blade is not rotating, but is subjected only to the forces imposed on the blade by the momentum of the gas flowing past the blade.

The phenomenon causing the erosion to be most severe at the intersection of the rotor blade with the platform supporting the blade is not fully understood, One explanation which would support the existence of a distributed erosion which increases in severity toward the axis of the rotor is that the relative velocity of the fluid and the entrained particles is higher toward the inside diameter of the rotor than toward the outside diameter. The extent of erosion is a function of the gas velocity, hence greater erosion takes place toward the rotor axis. This explanation, however, does not account for the concentration of erosion at the vane root.

The tips of the stator blades immediately upstream of the rotor vane roots and the clearance or leakage space between the stator tips and the rotor are undoubtedly the cause of one form of this erosion concentration. Since there is a pressure drop across the stator blades, it would appear that there would be a tendency for some gas on the high pressure side of the stator blade to be accelerated through the leakage space at the tip of the blade to the low pressure side. This accelerated flow, together with any entrained foreign particles, impinges at a high relative velocity upon the root of the succeeding rotor vane. Another theory is that the foreign particles tend to migrate toward the inside diameter along the boundary layers and trailing edge wakes of the stator blades. In any event, the concentrated erosion points do exist whether they are caused by increased relative velocities at this point or by increased quantities of foreign particles.

Although the foregoing discussion has been directed primarily toward gas turbines intended to burn a solid fuel such as coal, the construction to be described hereinafter is by no means limited to gas turbines, but is applicable also to steam turbines, for example, having liquid or solid In particular, the

use of saturated steam has become of increasing importance in recent years, for some applications, and consequently, erosion problems have become severe due to entrained water particles in the saturated steam. Therefore, the invention may also be applicable to steam turbines using saturated steam. It may also find application in axial flow compressors.

Accordingly, it is an object of the present invention to provide an improved turbo-machine bucket or blade construction which is designed to reduce erosion from foreign particles in the motive fluid in the vicinity of the vane root.

Another object is to provide an improved gas turbine rotor blade which is resistant to fly ash resulting from the combustion of a solid fuel.

A further object is to provide a turbo-machine rotor blade having an erosion resistant blade platform.

Still another object is to provide a rotor blade having a blade platform portion susceptible to erosion which is replaceable, rather than requiring replacement of the entire rotor blade.

Other objects and advantages will become apparent from the following description, taken in connection with the accompanying drawings, in which FIG. 1 is a longitudinal sectional view showing a portion of the rotor blade and stator blade arrangement in a turbine embodying the invention; and

FIG. 2 is an enlarged view taken along plane 2-2 of FIG. 1.

Generally stated, the invention is practiced by providing a stepped blade platform, with an elongated adjacent stator blade tip extending into a recess defined by the step, so that any motive fluid leaking around the stator blade tip impinges upon the radial surface of the blade platform rather than upon the root portion of the vane itself. The impingement surface of the platform is composed of a removable block of erosion-resistant material.

Referring now to the drawing, FiG..l shows portions of adjacent rotor Wheels I mounted for rotation inside a turbine casing 2. Mounted on each of the turbine wheels 1 is a row of radially extending rotor blades, one of which may be seen at 3 secured to wheel 1 by a fir tree dovetail t, or similar suitable fastening means. The method of fastening rotor blade 3 to the wheel 1 is not material to the present invention and any number of conventional securing means may be used.

Secured to the casing 2 by means of locking support portions 5 is a circumferential row of radially extending stator blades 6. The moving blades 3 and the stationary stator blades 6 cooperate toextract power from the motive fluid as it passes through the turbine in a manner well known to those skilled in the art, and although a small number of stator and rotor blades are shown in the drawing, there may be any number of stages of blading.

Each of the rotor buckets 3 is composed of a vane portion 3a, a platform portion 3b, and the aforementioned dovetail fastening portion 4. The platform portion 3b, in turn, is partially defined by an arcuate surface 30, from which extends the vane 3a, and by radial surfaces 3d, 32. Axially extending flanges 31, 3g having similar arcuate surfaces 3h, 3i, respectively, complete the platform 31).

Arcuate surfaces 3c, 3h, 31 extend circumferentially to similar arcuate surfaces 30, 3h, 31 on adjacent rotor blades 3 (looking into the plane of the drawing). 'Thus, surfaces 30 define a substantially continuous cylindrical surface coaxial with the rotor axis and similarly surfaces 3h, 3i form cylindrical surfaces of smaller diameter but coaxial with that defined by surfaces 30.

It may also be noted that the flanges 25 extend axially to meet similarly axially extending flanges 3g on another row of rotor blades 3, with only a small clearance gap 7 left between the flanges in order to prevent rubbing of the platforms, while restricting leakage of the motive fluid.

it will be seen that, in this manner, the blade platforms 3b provide a series of cylindrical stepped" surfaces of varying diameter forming the inner wall of the motive fluid flow path.

As will be understood by those skilled in the art, it is ordinarily the practice to have the stator blades 6 of generally the same length as the rotor blades 3 so that the tip 6a of the stator blade is directly opposite the vane root of the rotor blade 3. With this arrangement, it has been found that most severe erosion of the rotor blade occurs at the root portion identified 8 in FIG. 1.

In accordance with the invention, this erosion zone is moved inwardly, so as to lie opposite the platform portion 3b immediately above the flange 3]. To this end, stator blades 6 have a tip portion 6a which extends radially inward, substantially beyond the cylindrical surface defined by portions 3c of blade platforms 3b to form a clearance space 11 with flanges 31, 3g. Now the gas leaking around the tips 6a of the stator blades will impinge upon the radial surface 3d of the platform 31).

In keeping with the inventive concept here, a suitable hardened erosion shield is disposed so as to be impacted by the jet of leakage from the clearance space 11. To this end, a dovetail groove 9 is cut into the radial face 3d of the blade platform 3b. As seen more clearly in FIG. 2, the dovetail groove 9 extends all the way across the width of platform 3b.

A preshaped block 10 of erosion-resistant material is disposed in groove 9 so as to present a substantially radial face 10a to the oncoming motive fluid escaping through the clearance space 11 between stator blade tip 6a and the flange surfaces 3h, 31'. Block it as seen more clearly in FIG. 2, is shorter than the lengtn of groove 9 so that the edges 9a. of groove 9 at opposite ends of the groove may be peened to hold insert 19 in place in the groove.

Block 10 may be composed of any material which is sufficiently resistant to erosion which might be caused by foreign particles in the motive fluid, whether it be entrained particles of water in saturated steam, or entrained fly ash in the combustion gases from burning coal. A suitable material for insert it) which might be used in a coal-burning gas turbine would be cemented titanium carbide which would resist both the high temperatures encountered in gas turbines'of this nature and would also withstand erosion due to the fly ash. For lower temperature operation, an alloy such as Stellite might be used.

It is to be particularly noted that, due to the manner of securing block 10 to the blade platform 3b, the block may be a simple cast, sintered, molded, or extruded shape requiring no machining. This is very important, since generally speaking, materials having high erosion resistance are also extremely hard and brittle and difficult to machine. Consequently, block 19 may be formed by some process other than machining, such as being formed from compressed powder and later sintered or may be possibly formed by pressure molding or extrusion. Likewise, some ceramics may be applicable for the block 10.

The method of operation of my invention will be reasonably apparent from the foregoing description of the structure. The motive fluid containing foreign particles such as fly ash in the coal combustion gases, flows past the stator blades 6 and rotor blades 3 alternately. Although some erosion will occur on the vanes proper, there is found to be a tendency for increased erosion in the vicinity of the roots of the blades. With the blade platforms 3]: shaped so as to form annular surfaces of varying diameters, the tips 6a of the stator blades are disposed in the annular recesses formed by adjacent platforms. Thus the root of the rotor vane 3a indicated by arrows 8 is shielded from the increased local erosion occurring opposite the tips 6a of the stator blades. The gas containing fly ash which flows through clearance space 11 impinges upon the radial surface 10:: of erosion block 10, and the erosion which would otherwise occur is greatly reduced due to the hardness of the block.

The erosion shields 10 may conveniently be inserted during manufacture of the rotor blades 3 before they are assembled to the rotor wheels 1. The block is merely slid into the groove 9 and the ends of the groove peened as shown at 9a. It may be noted that the undercut surface 9b of groove 9 will produce a force component on block 16 due to centrifugal force as the rotor wheel 1 turns, which acts to retain the block securely in groove 9. In addition, the blocks 1% may readily be removed and replaced rather than replacing the entire blade 3.

As explained previously, this invention is also applicable to steam turbines and axial flow compressors, although, of course, the geometry of the blade and method of attachment to the rotor wheel will vary in accordance with differences due to the density of the motive fluid and the speed of rotation of the turbine wheels desired. It is, of course, desired to protect the inventive concept here no matter what sort of motive fluid is utilized or what type of foreign erosion-causing material is present in the motive fluid.

The invention is also applicable to the protection of the stator vane roots. The casing 2 would have to incorporate circumferential grooves to accommodate an extended portion of the rotor blade and the method of attachment of the erosion-resistant inserts would be in keeping with the method previously described in connection with the rotor blades. Such modifications in design will, of course, be apparent to those skilled in turbine design.

These and many other modifications will occur to those skilled in the art. It is intendedto cover by the appended claims all such modifications as fall within the scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A turbo-machine blade for use with a fluid having entrained particles of erosion-producing matter comprising a vane portion, a blade platform for supporting said vane portion and defining a first surface adjacent one end of the vane portion and disposed substantially normal to the vane axis, a shield member of erosion-resistant material secured to the blade platform on the side thereof below the first surface and having at least one surface disposed substantially normal to the first surface, whereby entrained particles can'impinge upon said erosion shield member rather than on the vane portions.

2. A turbo-machine rotor blade for use with a fluid containing erosion-producing foreign particles comprising a vane portion, a blade platform for supporting said vane portion, said platform defining a first arcuate surface adjacent one end of the vane portion and substantially normal to the vane axis and also having at least one flange portion defining a second arcuate surface disposed substantially parallel to and spaced below said first surface, and a shield member of erosion-resistant material secured to the blade platform between said first and second arcuate surfaces and having at least one surface facing the motive fluid flow, whereby entrained particles carried in the fluid along the second surface impinge upon said erosion shield member rather than on the vane portion.

3. An axial flow turbo-machine for use with a motive fluid containing erosion-producing particles comprising an annular casing having at least one circumferential row of radially extending stator blades, said stator blades having separated unshrouded tip portions, a rotor having at least two circumferential rows of rotor blades removably mounted on the rotor and said blades having radially extending vane portions arranged to cooperate with said stator blades, the rotor blades also having blade platforms integral therewith defining together with said casing the motive fluid flow path and each rotor blade platform defining a first surface adjacent and substantially normal to the rotor vanes extending circumferentially to form a substantially continuous outer cylindrical surface radially inward of the rotor vanes, the platforms also including axially extending flanges cooperating with an adjacent row of rotor blades to define a substantially continuous inner cylindrical surface of a lesser diameter than said outer cylindrical surface and disposed below the tips of the radially extending stator blades so as to form close clearances therewith, whereby motive fluid passing between the stator blade tips and said'second surface will impinge upon the blade platform rather than upon the vane portion.

4. An axial flow turbo-machine for use with a fluid containing erosion-producing particles comprising an annular casing having at least one circumferential row of radially extending stator blades, said stator blades having separated unshrouded tip portions, a rotor having at least two axially spaced circumferential rows of rotor blades with radially extending vane portions arranged to cooperate with said stator blades, said rotor blades also having blade platforms defining together with said casing a motive fluid flow path and each rotor blade platform defining a first arcuate surface adjacent and substantially normal to the rotor vanes extending circumferentially to form a substantially continuous outer cylindrical surface radially inward of the rotor vanes, the platforms also including axially extending arcuate flanges cooperating with an adjacent row of rotor blades to define a substantially continuous inner cylindrical surface of a lesser diameter than said first cylindrical surface and disposed below the tips of the radially extending stator blades so as to form close clearances therewith, an erosion-resistant shield member secured to the blade platform between said first and second arcuate surfaces to present a substantially radially extending surface to the motive fluid passing between the stator blade tips and the inner cylindrical surface, whereby motive fluid passing between said stator tips and the inner surface will impinge upon the erosion shield member rather than upon the vane portion. 5. A turbo-machine blade for use with a fluid having entrained particles of erosion-producing matter comprising a vane portion, a blade platform for supporting said vane portion, said platform defining a first arcuate surface adjacent one end of the vane portion and substantially normal to the vane axis and also having at least one flange portion on the front of the blade platform parallel to said first surface and spaced below said first surface, said platform also defining a dovetail groove located between said first arcuate surface and said flange portion, and a shield member of erosion-resistant material disposed in said dovetail groove but being of a shorter length than the groove so as to allow securing in place by peening the ends of the dovetail groove, whereby entrained particles carried in the fluid along the flange portion will impinge upon said xerosion shield member rather than'on thevane portion.

References Cited in the file of this patent UNITED STATES PATENTS 

